A passive ring laser gyroscope has an optical resonator cavity, usually in the form of a rectangle or a triangle, into which coherent light is introduced from an external laser source. An active ring laser gyroscope uses a gain-medium within a similar resonator cavity with a laser created therein.
Active ring laser gyroscopes are subject to problems of mode lock-in, bias drift due, for example, to gas flow, and other medium-related problems. These problems degrade the accuracy and precision of an active ring laser gyroscope. A passive ring laser gyroscope has been seen as an alternative to the active ring laser gyroscope as it has fewer problems.
Examples of passive ring laser gyroscopes include U.S. Pat. No. 4,135,822 by S. Ezekiel issued Jan. 23, 1979. The subject matter of this patent was also disclosed in an article: Ezekiel, S. and Balsamo, S. R. "Passive Ring Resonator Laser Gyroscope," Applied Physics Letters Vol. 30, No. 9, May 1977, pp. 478-480. The passive ring laser gyroscope described in the patent and the article uses a ring resonator with three or more mirrors as a rotation sensing element. An external laser is used to probe the resonator to determine the difference between the clockwise and anticlockwise cavity resonant frequencies. The difference arises from rotation of the cavity relative to the local inertial reference frame. The frequencies of the clockwise and counterclockwise travelling beams are maintained at their resonance frequencies by means of an electronic control system.
If two lasers are used, each is maintained at its respective cavity resonance frequency. In gyros utilizing a single source laser, the beam is split into two beams before being injected into the resonator cavity in the two directions. The frequency of each beam is independently controlled by acousto-optical devices, each driven by separate voltage-controlled oscillators (VCO). The shifted frequency beams exit the resonator cavity where two detectors are used to detect the respective frequency differences between the light beams and the cavity resonance frequencies. That is, one detector detects the frequency difference between the light travelling in the clockwise direction and the clockwise cavity resonance frequency. The other detector detects the frequency difference between the light travelling in the counterclockwise direction and the counterclockwise cavity resonance frequency. These two error signals are subtracted electronically providing a differential error signal which is proportional to the difference between the difference in optical frequencies and the difference in cavity resonance frequencies in opposing directions. The differential error signal is amplified and fed back for servo-control of a VCO which shifts the frequency of one of the counter rotating light beams. This electronic error balancing and feedback system of the prior art necessarily introduces electronic noise into the system. Problems of matching photodetectors make it difficult to achieve the stability needed to accurately measure rotation using a passive ring laser gyro design.
In a second article: Drever, R. W. P., Hall, J. L. and Kowalski, F. V., "Laser Phase and Frequency Stabilization Using Optical Resonator," Applied Physics B, Vol. 31, 1983, pp. 97-105, the authors disclose an optical frequency stabilization system which has an optical frequency discriminator and laser stabilization feedback configuration to achieve a longitudinal cavity resonator frequency which is substantially the same frequency as the coherent light source used in the system. In the Drever, et al. article, phase-modulated optical sidebands are applied to (or superimposed on) the carrier optical wave in order to measure the extent of adjustment of the resonator that is required to match its resonant frequency to the laser source frequency. A sole detector is used to detect the optical signal that, upon demodulation, provides an electronic error signal which can be electronically fed back to adjust the optical frequency of the laser so as to match that of the resonant cavity as determined by its length. This optically stabilized laser system has been operated only to achieve laser stabilization of a single laser frequency to a single cavity mode resonance frequency. It has not been operated to produce a differential signal detection and to derive a beat frequency indicative of rotation, as described by Ezekiel.
The electronics in the first of the prior art systems described above produce noise sources that limit the accuracy and precision of detected error signals and, thus, the beat frequency output needed to ascertain gyroscopic inertial rotation.